Numerous types of detection systems have been derived for detecting the presence of an object within a monitored zone. One type of detection system includes an emitter which transmits a pulse signal, such as an optical beam, into the monitored zone. A corresponding optical detector is oriented to detect the transmitted beam at an opposite side of the monitored zone. Alternatively, the optical detector may be positioned so as to detect a reflection of the transmitted beam from an object within the monitored zone. The detection of the transmitted beam is indicative of the presence or absence of an object within the monitored zone.
For example, U.S. Pat. No. 5,122,796 to Beggs et al. discloses a synchronous detection system having an electro-optical emitter that emits light into a monitored zone. An electro-optical receiver senses light reflected from an object within the monitored zone and provides a signal that is filtered by a synchronous detector. The synchronous detector excises a portion of the signal at a frequency based on the modulation frequency of the electro-optical emitter. Accordingly, the detector operates when a reflected pulse is expected, thereby screening out noise and other signals during intervals when no reflected pulse is expected.
U.S. Pat. No. 5,243,181 to Bondarev et al. discloses a synchronous detection system in which a transceiver emits light pulses into a monitored zone. The transceiver receives light pulses that are reflected back to the transceiver when an object is present. The transceiver, in turn, generates an output signal indicative of whether an object is present in a monitored zone. If the transceiver detects potentially interfering noise at a time other than when the light signal is emitted, such as detecting a noise level above a fixed detection threshold, then no pulse is generated for a preselected time period. The effects of noise also are reduced by tracking the number of pulses emitted but not received and by generating successive pulses at different frequencies.
U.S. Pat. No. 5,463,384 to Juds discloses an example of a collision avoidance system for a vehicle that utilizes a fixed threshold synchronous detection system. The disclosed systems includes a sensor for producing detection signals in response to receiving a reflected energy beam.
U.S. Pat. No. 4,851,661 to Everett, Jr. discloses a ranging system that includes a programmable array of optical emitters that are controlled to vary the intensity of emitted light. The system also includes an optical receiver having separately adjustable threshold detectors to facilitate a range determination of an object detected within a monitored zone.
In systems utilizing a fixed detection threshold, the threshold typically is selected based on the worst case expected noise condition to ensure a satisfactory of noise rejection. A consequence of this approach is to decrease the sensitivity of the system when the signal-to-noise ratio worsens. Conventional sensors often are expected to operate in widely varying or noisy environments, for example, in an environment with an electrical field of about 50 volts per meter over a frequency range from 10 MHz to about 1,000 GHz. Furthermore, performance expectations are increasing throughout the world for use of detection systems in the presence of impressed noise caused by external sources, such as variable frequency drives, load switching, static discharge, etc., such as expressed in the IEC 61000-4 series of standards.
In the example of a photoelectric detection system without additional ambient illumination, very little DC photocurrent is produced in the photodetectors, resulting in very little shot noise. However, when the system is operated under bright daylight conditions, there is significant DC photocurrent in the receiver photodetectors, which results in higher shot noise levels. The root mean square of DC photocurrent I.sub.n (RMS shot noise current) produced in a silicon photodiode is determined by the equation: I.sub.n =5.66 * 10.sup.-10 .sqroot.(I.sub.dc * BW); the units being amps rms, where I.sub.dc is the DC photocurrent and BW is the circuit bandwidth in Hz. Whenever electric current flows in the photodetector, such as when the photodetector views a white target in bright sunlight, the photocurrent generates shot noise at a level that is many times greater than the intrinsic electronic noise of the receiver amplifier itself. To avoid false detection caused by a high level of shot noise, the required threshold must be quite large in comparison to the worst case scenario for the shot noise. This high threshold, however, results in low system capability of detecting very dark, low reflective targets in all lighting conditions.
In order to address the deficiencies of fixed threshold systems, detection systems have been devised that adaptively adjust the threshold based on a measurement of the noise statistical characteristics. The noise measurement is used to set the detection threshold of the receiver. In this way, such adaptive systems are able to optimize their sensitivity relative to the ambient measured receiver noise levels in order to maintain adequate signal reception integrity. Examples of systems utilizing an adaptive threshold are disclosed in U.S. Pat. No. 3,999,083 to Bumgardner, U.S. Pat. No. 4,142,116 to Hardy, Jr. et al., U.S. Pat. No. 4,992,675 to Conner, Jr. et al., and U.S. Pat. No. 5,337,251 to Pastor.
It has also been suggested to statistically process signals in a detection system using filters having differing time constants. Each filter provides an output signal to a comparator that, in turn, determines the direction of a counter. The counter counts between limit values, such as between zero, which indicates a non-detect state, and N, which indicates a detect state, based on sampling the output of the comparator. The statistical nature of the processing algorithm requires that the peak signal and peak flyback pulse which are sampled at the output of the comparator be nominally of about the same magnitude. The output of the comparator is sampled at two points in time, indicated at S1 (e.g., corresponding to the peak signal) and at S2 (e.g., corresponding to the flyback pulse of the peak signal). Because the output of the comparator is either HIGH or LOW, the two sampling points provide four possible outcomes. Assuming a Gaussian distribution for noise, then the output of the detector when sampled at times S1 and S2 may be expected to be:
______________________________________ S1 S2 OUTPUT DESCRIPTION ______________________________________ LOW LOW -- Non-detect assumption causing count down LOW HIGH -- Non-detect assumption causing count down HIGH LOW + Correct detect decision causing count up HIGH HIGH -- Non-detect assumption causing count ______________________________________ down
Each state has a nominally 25% probability of being reached, and that the HIGH-LOW state statistically has greater than a 50% likelihood when there is a signal to detect. In this approach, the false detect signals and false non-detect signals, however, operate to lengthen the time to reach a true output state (e.g., zero or N). In addition, statistical assumptions have been made which require the peak signal at time S1 and the peak flyback signal pulse at time S2 to be nominally at about the same absolute amplitude.